1. Field of the Invention
The invention relates generally to the art of optical data transmission apparatus and methods and more specifically to a variable phase DC bias controller for an electro-optic modulator.
2. Description of Related Art
Interferometric electro-optic amplitude modulators, such as Mach-Zehnder and YBBM modulators, have recently become of great interest in the high density communications industry for high bandwidth data communications. An electro-optic modulator acts like a transducer of sorts that converts electrical energy or data into optical energy or data. A Mach-Zehnder modulator is one type of electro-optic modulator that enables a high bandwidth electrical signal to modulate an optical beam, which can then be transmitted over great distances using fiber optic cables. Electrical signals with bandwidths well into the GHz region can be used to modulate the optical signal carried over these cables.
Transmission of data using optical carriers enables, in addition to very high bandwidths, great numbers of mUltipiexed channels with low signal loss and distortion. In a typical electro-optic modulator, a laser light beam is amplitude modulated with a data signal, and propagates to a remote receiver either directly through the atmosphere, or via a system of optical fibers and repeaters. The light beam may advantageously be modulated with electrical signals in the microwave frequency range using an electro-optic modulator such as the Mach-Zehnder modulator. An electro-optic modulator based on a Mach-Zehnder interferometer generally includes a monolithic substrate formed of an electro-optic material such as LiNbO.sub.3 or GaAs. An optical waveguide is formed in the substrate having two arms or branches of equal length which extend generally in parallel to each other, wherein the index of refraction of the material in the waveguide is higher than the index of refraction of the material of the substrate.
In the absence of an applied electrical bias voltage, an optical input to the waveguide divides the laser light equally between the branches. Optical signals propagating through the branches can recombine at the optical output of the waveguide in phase or out of phase with each other. An electrical bias voltage differential applied to one branch of the waveguide relative to the other branch causes the indices of refraction of the material in the branches to vary differently due to electro-optic effects, thereby changing the effective optical lengths of the branches. At a bias voltage known in the art as V.sub..pi., the effective optical lengths are varied to such an extent that the optical signals emerging from the branches are 180.degree. out of phase, with each other. V.sub..pi. is the voltage required to go from a minimum to a maximum in optical output.
Prior art optical communication applications have been carried out at a V.sub..pi. /2 voltage which produces a preferred linear operation at that point, and indeed efforts have been made to keep that point from drifting due to device instabilities and environmental effects, especially temperature variations. The linear bias point had to be maintained during any link operation to achieve maximum dynamic range since second order harmonic and intermodulation distortion (fringes) increase rapidly with increasing bias voltage error. There exists a need, and it would be preferable to operate an integrated optical Mach-Zehnder modulator at virtually any point of the sinusoidal optical output curve. All prior art DC bias applications were oriented to maintaining modulation at the V.sub..pi. /2 point of the optical curve, which does not give the desired flexibility to modulate anywhere on the output curve. The invention disclosed herein conveniently dispenses with the foregoing limitations.